This invention relates to a novel process for the preparation of halogen-containing organophosphorus acid esters wherein halogen is directly bonded to phosphorus. More particularly, this invention is concerned with a process comprising an amine catalyzed reaction of halides of phosphorus and hydroxyl-containing organic materials, i.e. aromatic alcohols.
Numerous methods have been long known for preparing triorganphosphorus esters. One of those methods involves the reaction of a phosphoryl halide and a monohydric organic compound without the use of a catalyst. Such a process is not commercially practical because of the need for lengthy reaction times and the resultant low yields. Another disadvantage of processes of that type is the need for the use of excessive amounts of the monohydric organic compound.
Another known method comprises the addition of certain amines to the aforedescribed reaction mixture to effect higher yields. Thus, U.S. Pat. No. 1,785,951 discloses the use of certain aromatic amines, e.g., aniline and pyridine, as catalysts for preparing triaryl phosphates from phosphoryl chloride and a phenol at high temperatures. Similarly, U.S. Pat. No. 2,678,940 discloses the use of aromatic primary amines, e.g., aniline, and certain tertiary alkylamines, e.g., trimethylamine, as catalysts for preparing triaryl phosphites from phosphorus trichloride and a phenol. The cited processes, however, are concerned only with the preparation of triaryl phosphates or phosphites and not with the selective preparation of halogen-containing mono- and di-esters of phosphorus acids.
In still other methods, the reaction is catalyzed, so as to produce greater yields, by adding a metal to the reaction mixture, such as copper powder, iron filings, calcium, aluminum or magnesium; or a halide such as aluminum chloride, magnesium chloride or boron trifluoride; or a sulfate such as copper sulfate; or an oxide such as magnesium oxide or copper oxide.
The employment of such catalysts has several attendant inherent disadvantages, among which are low conversion of the starting materials and lengthly reaction times required for completion of the reaction. As described in U.S. Pat. Nos. 2,610,978 and 2,632,018, an insoluble complex forms during the reaction when aluminum chloride is used as a catalyst.
When alcohols are reacted with a phosphoryl halide, either without a catalyst or in the presence of any of the above-mentioned catalysts, other than magnesium chloride, undesirable by-products are formed. The by-products contribute difficult distillation problems, lower yields of the desired product and lower reaction efficiency. A method described in U.S. Pat. No. 2,410,118 is illustrative of the typical distillation problems encountered. In that method, distillation is difficult due to the high concentration of salts of various phosphorus acids in the distillation still.
U.S. Pat. No. 2,868,827 describes the use of titanium tetrachloride as a catalyst for producing organophosphate esters. Disadvantages encountered employing titanium tetrachloride reside in the excessive and lengthy times necessary to obtain desirable yields and the relatively large amounts of the metal halide catalysts required. Further, when the reaction is conducted in the presence of a titanium halide catalyst, recovery of the desired reaction product is a problem. At the completion of the reaction, it has been found necessary to wash the reaction mixture with a citrate or tartrate solution which forms a complex with the titanium catalyst. The complex is then removed by washing with water followed by drying the remaining product.
Another disadvantage encountered with the employment of many of the aforedescribed catalysts is the need for complicated material-handling procedures for the catalyst.
Additionally, preparation of organophosphorus esters by the aforedescribed catalyzed reactions restricts the manufacturer in that only one specific type of organophosphorus ester of high purity could be prepared by the reaction. Thus, one could not prepare compounds of high purity such as, for example, cresyl phenyl phosphorochloridate, bromophenyl phenyl phosphorochloridate and the like. By the aforedescribed procedures, only relatively impure triorganophosphorus esters or esters containing the same aryl groups could be prepared, for example, triphenyl phosphate, tricresyl phosphate and the like. Thus, in the preparation of organophosphorus esters by the aforedescribed procedures, the specific organophosphorus esters prepared were contaminated by side reactants which could only be removed by lengthy and difficult washing and distillation procedures.
Accordingly, the objective of this invention is to provide a novel and improved process for the preparation of mono- and diorganophosphorus esters in which the disadvantages of the prior art are eliminated and selective esterification in high yields with substantially no by-product contamination is afforded.